Sugar-induced molten-globule model

Proteins denature at low pH because of intramolecular electrostatic repulsi ons, The addition of salt partially overcomes this repulsion for some prote ins, yielding a collapsed conformation called the A-state. A-states have ch aracteristics expected for the molten globule, a notional kinetic protein f olding intermediate. Here we show that the addition of neutral sugars to so lutions of acid-denatured equine ferricytochrome c induces formation of the A-state in the absence of added salt. We characterized the structure and s tability of the sugar-induced A-state with circular dichroism spectropolari metry (CD) and NMR-monitored hydrogen-deuterium exchange experiments. We al so examined the stability of the sugar-induced A-state as a function of sug ar size and concentration. The results are interpreted using several models and we conclude that the stabilizing effect is consistent with increased s teric repulsion between the protein and the sugar solutions.